TECHNICAL | HYDRO POWER CAVERNS


likelihood (vertical) and impact (horizontal)


120000 100000 80000 60000 40000 20000 0


0 6 Observations 4 2 0


12 4 Layered


12 10 8 6 4 2 0


6 Magmatic 8 Log. (layered) 10 1214 Log. (magmatic)


Strength/stress ratio Average ~5.20


Excavation volume Vmean = 57,000 m3


V conv. = 27C + 7600 V PSP = 12C + 39000


number. An ideal distribution would be that the centre/ average risks should be occasional and serious (or 3+3 and clearly acceptable). However, likely and considerable is more frequent and conservative average risk grade.


600 1200 1800


2400 Total installed capacity C (MW)


15x pumped storage <1,800 MW 3000 3600


4200


6. POSSIBLE RISK MITIGATION EFFECTS Ten simplified mitigation groups have been previously studied to distinguish between the extreme grades of very low and very high impact: These mitigation groups are as follows: a Improved communication (or methods) b Change order and new contract items c Additional field investigations d Re-analysis and re-design e Independent second (check-) engineer f Improved monitoring measures g Additional laboratory investigations h New contractor/engineer (rarely) i Increased work capacities/resources j Employment of specialised sub-contractor


Risk mitigation effects for 50 sites, supported by waterways risk examples and various design reports, were evaluated: Both grades reduced 20-25% Impact is reduced


40-45%


Likelihood is reduced 15-25% No grade is reduced 10-20%


Observations


High water influence once at all 3 groups


5 3 2 2 1 2 1 2 1


Technical risk grades observed compared to contractual risks including failure at works are as follows: adverse geological changes are a higher probability but with moderate impact, and poor performance or risk of failure at the construction site create moderate probability but higher impact. The reason for this may be that for most reported case histories a geotechnical investigation was adequate and timely. In less favorable cases geotechnical risk grades may be clased as contractual. In 2015 Hudson & Feng12


compiled a geotechnical 33/18 Sedim. 35/18 Magmat. Top, figure 4:


Transformer cavern volume and installed capacity: evaluation of 10 conventional and 15 pumped storage plants Centre, figure 5:


Intact rock compressive strength and maximum cavern contour stress ratio: evaluation of 18 sedimentary and 18 magmatic host rock plants in China Above, figure 6:


Cavern group failure modes: evaluation of 46 caverns and 88 failure modes in China 19/10 others


database of 40-60 different hydro cavern examples. By evaluating these mainly large conventional hydro caverns in China several correlations/trends are supplemented: 1 Total installed capacity, specific unit capacity, and unit numbers are separated for conventional and pumped storage plants.


2 Simplified bedded or non-bedded or 4-5 host rock formation groups for conventional or pumped storage plants.


3 Cavern group rock pillar widths and transformer cavern heights related to power cavern heights.


4 Transformer cavern or surge chamber volumes and installed total plant capacity.


5 Ratio of intact rock compressive strength and maximum cavern contour stress.


6 Rock mass failure modes for cavern groups for bedded and non-bedded rocks.


20 | September 2023


Interlayers Rock blocks Fault zones Var. stress Rock burst Displacement Poor blast Poor quality Group effect


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